This invention relates to a transfer device for transferring a toner image formed on an image forming body to a copying medium and in particular to a transfer device for use in an electrostatic copying apparatus.
In almost all electrostatic copying apparatuses currently available, direct transferring by the corona charger is used to form a non-contacting type device. A conventional method is known in which as shown in FIG. 1 a corona charger 12 is disposed opposite to the surface of an endless belt 10 to permit the insulating belt 10 to be charged by the corona charger 12. The charged belt 10 is pressed by a compression roller 14 against the surface of a photosensitive drum 16. In this state, a copying paper is sandwiched between the photosensitive drum 16 and the insulating belt 10 to cause a toner image on the surface of the photosensitive drum 16 to be transferred to the copying paper.
Use may also be made of a transfer device as shown in FIG. 2 in which a corona charger 12 is disposed opposite to the rear surface of the insulating belt 10.
In either transfer device, a high voltage is required for the corona charging of the corona charges, involving a lot of risk. During the corona charging, ozone is generated, causing a possible air contamination as well as a possible contamination of the inner mechanism of the electrostatic copying apparatus. Further, the corona charge 12 is expensive and provides a bar to the cost reduction of the electrostatic copying apparatus.
In the above-mentioned transfer operation, on the other hand, the insulating belt 10 is electrically charged. Such amount of charge is cumulated for each copying operation, resulting in a change in amount of charge per each copying operation. This necessitates providing a discharger for discharging the charged insulating belt for each copying operation.